To target genes in mice, the construct is inserted into mouse embryonic stem cells in tissue culture. Cells with the correct insertion can be selected, because they carry a visible marker (such as a gene for white fur where the normal is brown). The treated cells can be added a mouse's tissue by embryo injection. Finally, chimeric mice where the modified cells make up the reproductive organs are selected for breeding. After breeding to others of the same kind, the entire body of the offspring mice has the altered gene.

In practice, large numbers of the injected mice are mated, which produces a few white mice (this only occurs when there are white-type cells in the germ line). Cross-breeding the white mice produces 50% of offspring which are homozygous for the marker, and so pure-bred for the experimental gene being inserted. Tests on the mice with the gene insertion can now take place.